Laser gyroscope system

ABSTRACT

A four-frequency laser gyroscope system having improved accuracy is constructed using a single solid block of low thermal coefficient of expansion material. A four-segment nonplanar propagation path provides a first frequency splitting. A second splitting is provided by a Faraday rotator having a thin slab of rare earth-doped glass positioned within an aperture in a permanent magnet. A narrow angle of incidence is provided for the beams of incident upon the output mirror to prevent cross coupling between beams within the output optics structure. Blocking the gaseous flow path reduces output frequency drift caused by contamininating particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 868,096, Filed Jan. 3, 1978now abandoned.

This application contains subject matter in common with U.S. patentapplication Nos. 646,307, now abandoned and 646,308 filed Jan. 2, 1976,now U.S. Pat. No. 4,110,045 the benefit of the filing date being herebyclaimed for the common subject matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains broadly to the field of laser gyroscopes. Moreparticularly, the invention pertains to four-frequency laser gyroscopesystems having effectively two laser gyroscopes operating simultaneouslywith a common propagation path.

2. Description of the Prior Art

The operation of a basic four-frequency laser gyroscope is described inU.S. Pat. No. 3,741,657 issued June 26, 1973 to K. Andringa and assignedto the present assignee. In such systems as described in the referencedpatent, waves of four distinct frequencies propagate around a closedpropagation path defined by three or more mirrors. Two of these beamscirculate around the closed propagation path in the clockwise directionwhile the other two circulate in the counterclockwise direction. One ofthe clockwise beams and one of the counterclockwise beams are of a firstpolarization sense while the other one of the clockwise and the otherone of the counterclockwise beams are of another polarization sense. Forexample, the first clockwise beam and first counterclockwise beam may beof right-hand circular polarization while the second clockwise andsecond counterclockwise beams may be of a left-hand circularpolarization. The two right-hand circularly polarized beams may be, forexample, of the highest two frequencies while the two left-handcircularly polarized beams may be of the lowest two frequencies.

Rotation of the laser gyroscope about its sensitive axis causes the tworight-hand circularly polarized beams to become further apart infrequency than at the rest state while the two left-hand circularlypolarized beams become closer together in frequency. The oppositefrequency shifts occur for the opposite direction of rotation. As shownin the referenced path, the difference between the frequency shifts inthe right-hand circularly polarized beams and the left-hand circularlypolarized beams is directly proportional to the rate of rotation of thesystem. The time integral of this difference is directly proportional tothe total amount of rotation about the sensitive axis.

Two separate means are provided within the propagation path forproducing frequency splitting in order to maintain the beams of fourseparate frequencies. In the system described in the referenced patent,a crystal rotator provides a split between the average of thefrequencies of the right and left-hand circularly polarized beams. Thissplit is accomplished by the crystal providing a phase delay forcircularly polarized waves that is different for one sense of circularpolarization than for the opposite sense and is reciprocal. A Faradayrotator further provides the frequency split between the frequencies ofthe clockwise and counterclockwise beams of like polarization. TheFaraday rotator is non-reciprocal providing different phase delays forwaves of the same polarization states propagating in oppositedirections.

Although the system of the referenced patent has been found to functionquite well, it has been found desirable to provide still furtherimprovements. For example, it is desirable to eliminate as much solidmaterial from the propagation path as possible as presence of any solidmaterial within the path provides scattering centers from which lightmay be undesirably coupled from one beam to another thereby inducingoutput frequency drift into the system. Furthermore, it is desirable toprovide a laser gyroscope system in which the coupling between beams ofthe opposite sense of polarization at the output detector issubstantially eliminated.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a lasergyroscope system having a minimum amount of material and hencescattering sites disposed within the propagation path of the circulatingbeams.

Furthermore, it is an object of the invention to provide a lasergyroscope in which unwanted coupling between beams incident upon anoutput detector are minimized.

Also, it is an object of the present invention to provide a lasergyroscope system in which drift due to flow of the gaseous gain mediumis minimized. These, as well as other objects of the invention, may bemet by providing the combination of means for providing a closednonplanar propagation path sustaining electromagnetic waves and meansfor producing an indication of the rate of rotation of said pathproviding means. As herein used the term closed propagation path relatesto a re-entrant path having a nonzero area projected upon some plane.The indication is preferably in the form of one or more electricalsignals which have a parameter such as frequency or amplitude whichvaries in accordance with the rate of rotation. Digital signals may beso employed. Waves of at least two distinct frequencies propagate aroundthe closed path. The indication means may produce a signal having afrequency substantially proportional to the difference in frequencybetween at least two of said waves. If waves of four frequencies areused, the indication may be in proportion to the difference between twodifferences between a separate two of the waves. In preferredembodiments, the waves are substantially circularly polarized. Four ormore reflecting means may be used to provide the closed path. Objects ofthe invention may further be met by providing the combination of aclosed nonplanar propagation for electromagnetic waves and means forproviding different delays for waves propagating in opposed directionsaround said closed path. Means may also be provided for extracting aportion of the waves propagating around said closed path and forproducing at least one output signal in response to the extractedportion of the waves. In one preferred embodiment, the means forproviding different delays may comprise a Faraday rotator.

In a preferred embodiment the invention may be practiced with thecombination of means for providing a closed nonplanar propagation pathfor electromagnetic waves and means disposed in the path for delayingwaves propagating in different directions along the path by differentamounts of time, the delaying means comprising a slab of material havinga thickness of less than 0.5 mm. Means should be provided for producinga magnetic field within the slab. The slab material has a preferredVerdet constant in excess of 0.25 min./cm. Oe. the operating wavelength.A glass with an appropriate rare-earth dopant will fulfill this purpose.Also, objects of the invention may be met by providing the combinationof means for forming a closed path for propagation of electromagneticwaves and means for coupling a portion of the waves out of the path withthe waves incident upon the coupling means having an angle between themof thirty degrees or less. The means for forming the closed pathpreferably comprises a block of solid material having a plurality ofpassages provided therein along which the electromagnetic waves maypropagate. Reflecting means are positioned at the intersections of thepassages. One of the reflecting means may be partially transmitting forperforming the function of coupling a portion of the waves out of thepath. Preferably, the closed path is nonplanar; that is, the varioussegments of the closed path do not lie within a single plane.

Moreover, objects of the invention may be met by providing thecombination of a plurality of reflecting means which provide a closedpath for propagation of electromagnetic waves with the path havingstraight line segments between the reflecting means with one of thereflecting means being partially transmitting and means for producingone or more electrical signals in response to the electromagnetic wavespropagating along the closed path wherein the signal producing meansoperates on portions of the electromagnetic waves transmitted by thepartially transmitting one of the reflecting means with the anglebetween one of the electromagnetic waves incident upon the partiallytransmitting reflecting means being thirty degrees or less. Thecombination may further include means for delaying waves propagating inone direction along the path by a different amount of time than wavestraveling in the other direction along the path. The delaying means maybe a Faraday rotator. Preferably, the path is nonplanar and is providedwithin a block of solid material.

The invention may also be practiced by providing the combination of ablock of solid material having a low thermal coefficient of expansion aplurality of straight passages being provided within the blockintersecting one another to form a closed path for propagation forelectromagnetic waves, a plurality of reflecting means one of which ispositioned at each intersection between the passages to reflect theelectromagnetic waves along the closed path with at least one reflectingmeans being partially transmitting with the angle between the passagesintersecting at the partially transmitting one of the reflecting meansbeing thirty degrees or less, and means for producing output signals inresponse to portions of the electromagnetic waves transmitted throughthe partially transmitting one of the reflecting means. The closed pathis again preferably nonplanar providing an image rotation for theelectromagnetic waves. The intersections of the passages are at thesurface of the block.

The invention may otherwise be practiced by providing the combination ofa block of solid material having a low thermal coefficent of expansionand a plurality of straight line passages within the block intersectingone another to form a nonplanar closed path for propagation ofelectromagnetic waves within the block with a plurality of reflectingmeans one of which is positioned at each intersection between thepassages to reflect the electromagnetic waves between the passages withthe reflecting means providing rotation for electromagnetic waves withinthe path. A Faraday rotator is disposed within the path. The Faradayrotator preferably comprises a thin slab of rare earth-doped glass thethickness of the crystal being less than 0.5 mm and means for providinga longitudinal magnetic field within the slab. The intersections betweenthe passages are located upon the surfaces of the block. Each of thesurfaces of the block in a plane perpendicular to a line bisecting theangle formed between the ones of the passages intersecting at each ofthe surfaces. A laser gain medium such as a gas mixture consisting of,for example, 8 parts ³ He to 0.53 parts .sup. 20 Ne to 0.47 parts ²² Neat a total pressure of 3 torr, should also be provided within the closedpath. A plurality of electrode means for exciting the laser gain mediumare also provided.

The objects of the invention can further be met by providing thecombination of a block of solid material having at least one first boretherein for propagation of electromagnetic waves with the first borehaving first and second colinear portions with the first portion lyingbetween a surface of the block and the second portion and with the firsthaving a larger cross section than the second portion, reflecting meanspositioned at the intersection of the first portion of the first borewith the surface of the block, and at least one electrode positioned ina second bore intersecting the second portion of the first bore. Thedistance between the intersection of the second bore with the secondportion of the first bore to the intersection of the first and secondportion of the first bore is preferably less than twice the diameter ofthe second portion of the first bore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top isometric view taken from a first corner of a lasergyroscope system of the invention;

FIG. 2 is a lower isometric view taken from a second corner of thedevice shown in FIG. 1;

FIGS. 3 and 4 are isometric views of the gyro block of the system shownin FIG. 1 showing the internal construction and passages of the devicetherein;

FIG. 5 is a cross-sectional view showing the internal construction ofthe system shown in FIG. 1 in the region of one of the terminal chambersand mirror substrate;

FIG. 6 is a cross-sectional view showing the details of construction ofthe Faraday rotator device of the laser gyro system shown in FIG. 1;

FIG. 6A is a cross-sectional view showing portions of the Faradayrotator of FIG. 6;

FIG. 6B is a cross-sectional view showing portions of another embodimentof the Faraday rotator of FIG. 6A;

FIG. 7 is a graph showing the gain versus frequency of the gaseous lasermedium employed with the laser gyro system of FIG. 1 indicating therelative positions of the frequencies of the four beams within thesystem; and

FIG. 8 is a graph showing the power reduction factor as a function ofthe angle of incidence of beams upon an output mirror structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring simultaneously to the views of FIGS. 1-4, the construction andoperation of a laser gyroscope system in accordance with the teachingsof the present invention will be described. Gyro block 102 forms theframe upon which the system is constructed. Gyro block 102 is preferablyconstructed with a material having a low thermal coefficient ofexpansion such as a glass-ceramic material to minimize the effects oftemperature change upon the laser gyroscope system. A preferredcommercially available material is sold under the name of Cer-Vit™material C-101 by Owens-Illinois Company.

Gyro block 102 has nine substantially planar faces as shown in thevarious views of FIGS. 1-4. As shown most clearly in the views of FIGS.3 and 4 which show gyro block 102 without the other components of thesystem, passages 108, 110, 112 and 114 are provided between four of thefaces of gyro block 102. The passages define a nonplanar closedpropagation path within laser gyro block 102.

Mirrors are provided upon faces 122, 124, 126 and 128, at theintersection of the passages with the faces. Substrates 140 and 142having suitable reflecting surfaces provide the mirrors positioned uponfaces 124 and 126 respectively. A mirrored surface is also provideddirectly adjacent face 128 in the front of path length controltransducer 160. One of these mirrors should be concave to insure thatthe beams are stable and confined essentially to the center of thepassages. Also, a partially transmitting mirror is provided upon face122 to allow a portion of each beam traveling along the closed pathwithin the gyro block 102 to be coupled into output optics 144. Thestructure of output optics 144 is disclosed in U.S. patent applicationSer. No. 758,223 filed on Jan. 10, 1977 by the present inventors andassigned to the present assignee.

Because passages 108, 110, 112 and 114 define a nonplanar propagationpath for the various beams within the system, each beam undergoes apolarization rotation as it passes around the closed path. Only beams ofsubstantially circular polarization can exist in the nonplanar cavity ofthe invention. With circularly polarized beams, drift due to beamscattering or coupling from one beam to the other is minimized. Thisreduction occurs because light of one circular polarization state whenscattered is not of the proper polarization to be coupled into andaffect the other beams. For other types of light polarization this isnot the case because there will always be some component of thescattered beam which will couple to other beams.

In the preferred embodiment, the passages and reflecting mirrors are soarranged as to provide a substantially ninety-degree polarizationrotation for the various beams. Because beams of right and left-handcircular polarization are rotated in opposite senses by this same amountindependent of their direction of propagaton, a frequency splittingbetween beams of right and left-hand circular polarization must occur inorder for the beams to resonate within the optical cavity. This is shownin FIG. 7 as the frequency split between the beams of left-hand andright-hand circular polarization. In the preferred embodiment, aninety-degree rotation corresponding to a 180-degree relative phaseshift is employed although other phase shifts as well may be useddepending upon the frequency separation desired. Rotation will occur aslong as the closed propagation path is nonplanar. The precisearrangement of the paths will determine the amount of rotation.

In the known systems of the prior art such as that described in theabove-referenced patent to K. Andringa, the frequency splitting betweenbeams of right and left-hand circular polarization was accomplished withthe use of a block of solid material of significant optical thicknessdisposed in the propagation path. As discussed above, the presence ofany solid material directly in the path of beam propagation providesscattering centers from which light may undesirably be coupled from onebeam to another causing an error in the gyro output. The amount ofcoupling and thus error is thermally very sensitive. Hence, the outputfrequency of such devices was subject to a temperature dependent driftwhich could not be compensated for with a fixed output bias. With thepresent invention, the solid material which had been used for thecrystal rotator has been completely eliminated from the beam propagationpath thereby eliminating the sources of error and drift associated withthe material.

To aid in understanding how the phase shift occurs, it is useful toimagine a linearly polarized beam propagating around the path. Suppose,for example, that the beam traveling between face 122 and face 124 islinearly polarized with the electric vector pointing in the upperdirection. As the beam is reflected from the mirror provided upon face124 the electric vector is still nearly pointed upward but with a slightforward tilt because passage 112 drops between face 124 and face 128. Asthe beam is reflected from the mirror upon face 128 it will be pointingnearly to the left with a slight downward tilt as would be seen in FIGS.3 and 4. As the beam is reflected from face 151, the electric vector ofthe beam within passage 108 would point to the left with a slight upwardslope again in the views of FIGS. 3 and 4. Thus, it may be seen that thebeam as it arrives back at face 122 has experienced a polarizationrotation of approximately ninety degrees. Of course, such a rotatedlinearly polarized beam cannot reinforce itself and resonate along theclosed path. Only circularly polarized beams having a frequency shiftedfrom the frequency at which such beams would resonate for a planarclosed path of the same length will be resonant.

A two-frequency laser gyroscope may be constructed using a nonplanarpropagation path to provide the only frequency splitting. No Faradayrotator or other such element is required in such an embodiment. Todetect the rate of rotation, an output signal is produced by beating theextracted portions of the two beams together to form an output signalhaving a frequency equal to the difference in frequency between the twobeams. At rest, the output signal will remain at some value f_(o). Forrotation in one direction the output signal will increase to a valuef_(o) +Δf, where Δf is proportional to the rate of rotation, and willdecrease to a value of f_(o) -Δf for rotation in the other direction.Use of circularly polarized waves in accordance with the inventionsignificantly reduces cross-coupling due to backscattering so that thelock-in range diminishes permitting such a laser gyroscope to be used inmany applications without complete elimination of lock in.

The second frequency splitting between the clockwise andcounterclockwise beams is caused by Faraday rotator 156. Faraday rotator156 is positioned within an aperture in face 151 as may be seen in theviews of FIGS. 2 and 4. The details of the construction of Faradayrotator 156 are seen in the views of FIGS. 6 and 6A. The Faraday rotatormount 154, preferably formed of the same material as laser gyro block102, forms the base upon which the structure is constructed. Faradayrotator mount 154 has a central cylindrical portion with one end flangedto restrain lateral movement of the device within aperture 120 providedin laser gyro block 102. The other end of Faraday rotator mount 154 iscut away to leave a platform for mounting the active components.Aperture 155 is provided aligned with passage 112 and havingsubstantially the same diameter as passage 112. Permanent magnet 166, ofhollow cylindrical shape, is positioned around aperture 155. Within theaperture in permanent magnet 166 is aperture 155. Within the aperture inpermanent magnet 166 is positioned slightly wedge-shaped Faraday rotatorslab 165. As shown in FIGS. 6 and 6A, the cross-section of the secondend portion, with both the permanent magnet and slab mounted thereon, isno greater than the cross-section of the central portion of the basemember. Faraday rotator slab 165 may be preferably formed of a rareearth-doped glass or a similar high Verdet constant material. A Verdetconstant of magnitude in excess of 0.25 min./cm./Oe. at the operatingwavelength is preferred to reduce the thickness of the slab required toproduce the desired amount of frequency splitting. It is desirable touse as thin a slab as possible because the amount of thermally induceddrift in the output of the device has been found to be a strong positivefunction of the thickness of solid material in the path of the waves. Acommercially available material is Hoya Optics, Inc. material no. FR-5.A thickness of 0.5 mm or less is preferred to reduce drift to anacceptable level.

Faraday rotator slab 165 is held against Faraday rotator mount 154 bycoil spring 168. Pole piece 170, which is formed of unmagnetizedferromagnetic material, is held against permanent magnet 166 by themagnetic field of permanent magnet 166. Pole piece 170 has an aperturein the center thereof of substantially the same diameter as that ofaperture 155 and passage 112 which is of slightly smaller diamater thanthe aperture within permanent magnet 166. Coil spring 168 is thusrestrained by the portion of pole piece 170 extending within theaperture in permanent magnet 166.

In the alternate embodiment of FIG. 6B, two cylindrical permanentmagnets 172 and 176 are positioned end-to-end with like poles adjacentone another at the juncture between the two magnets. The Faraday rotatorslab 165 is placed adjacent one end of the two magnet pair. Alongitudinal magnetic field is produced in the slab but this fieldattenuates rapidly upon moving a short distance away from the slab ormagnets. This embodiment has the advantage that essentially no straymagnetic field is produced which could extend into the gaseous dischargeregion and, by the Zeeman effect, produce unwanted modes or frequencyoffset.

Besides providing the frequency splitting between the clockwise andcounterclockwise circulating beams, Faraday rotator 156 performs asecond function. Because of the close fit provided within aperture 120in gyro block 102, Faraday rotator 156 blocks the longitudinal flow ofgas through passage 112. Because there can be no net circulation of gasthrough the closed path, the possibility of circulation of scatterparticles carried by the gas is substantially reduced. Both surfaces ofFaraday rotator slab 165 are preferably provided with an anti-reflectioncoating to prevent backscattering of the incident radiation. Also, somereflection may be permitted with the reflected radiation utilized forthe output signal. A partially transmitting mirror is not then required.

Referring again to the views of FIGS. 1, 3 and 4, it may be seen that alow angle of incidence is provided for the beams striking the partiallytransmitting mirror disposed upon face 122. The beams traveling withineach passage 108, 110, 112 and 114 are generally circularly polarized.The nearer to normal that one of these beams strikes a reflecting mirroror a surface the nearer to circular will be the polarization of the beamtransmitted through the mirror surface. As the angle of incidence movesaway from the normal the paritally transmitted beams begin to assume anelliptical polarization.

As explained in the above-referenced U.S. patent application No.758,223, if the beams within the output optics and detector structureare entirely circularly polarized there will be essentially no unwantedcross coupling and interference between the beams of the upper twofrequencies and the beams of the lower two frequencies within thedetector structure. As the amount of ellipticity increases, crosscoupling begins to become evident and appears as an amplitude modulationupon the output signals from detector diodes 143. It has been discoveredthat the amount of the unwanted cross coupling is a nonlinearmonotonically increasing function of the degree of ellipticity. It hasbeen found that the cross coupling is relatively low for angles ofincidence below approximately fifteen degrees. However, the amount ofcross coupling increases quite rapidly above this angle of incidence.This cross coupling may be eliminated by means of a suitablepolarization filter, but the available filtered power decreases as theunfiltered cross coupling increases.

Furthermore, as the angle of incidence of each beam upon the outputmirror increases, the filtered power available at the detector diodesfor each beam decreases. A calculated graph of power reduction factor,the ratio of power available at the detectors at a given angle ofincidence to that available for the same beam normal to the mirrorsurface, is shown in FIG. 8 for the output structure described in theabove-referenced U.S. patent application No. 758,223. As may readily beseen, the power reduction factor falls rapidly for angles of incidencesgreater than approximately fifteen degrees. Hence, in accordance withone aspect of the invention, the angle of incidence of the beams inpassages 108 and 110 to the partially transmitting mirror disposed uponface 122 is made to be fifteen degrees or less. Alternately stated, theangle between passages 108 and 110 is thirty degrees or less.

Still referring to the views of FIGS. 1, 3 and 4, electrodes forexciting the gaseous gain medium disposed within passages 108 and 110are positioned within electrode apertures 104. Preferably, centercathode electrodes 132 and 136 are connected to the negative terminal ofan external power supply while electrodes 127, 130, 134 and 138 areconnected to the positive terminal. The cathode electrodes are in theform of hollow metal cylinders capped at the end most distant from theseal to the laser gyro block 102 while the positive electrodes are inthe form of metal rods extending into the various electrode apertures104. With this connection, the current flows outward toward electrodes132 and 136 in two opposed directions within a single passage. Negativeelectrode 136 is preferably located midway between positive electrodes134 and 138 as negative electrode 132 is located midway between positiveelectrodes 130 and 127. In this manner, because a beam traversing one ofthe passages in which the electrodes are located passes through equallengths of current flow of opposite direction, the effects of drag onthe beam caused by unequal current flow through the gaseous gain mediumare substantially eliminated. However, because of manufacturingtolerances in the positions of the various electrodes the distancesbetween the positive and two negative electrodes in each passage may notbe precisely equal. To compensate for the inequality, the current flowbetween the positive electrodes and thereto adjacent negative electrodesmay be made unequal.

The gaseous gain medium which fills passages 108, 110, 112 and 114 issupplied through gas fill aperture 106 through gas fill tube 146 from anexternal gas source. A mixture ³ He, ²⁰ Ne and ²² Ne in the ratio of8:0.53:0.47 is preferred.

The details of construction of the laser gyroscope system in the regionof one of the positive electrodes are shown in detail in thecross-sectional view of FIG. 5. Metal electrode 130, held in place byglass electrode seal 131, is positioned within electrode aperture 104.Electrode 130 extends somewhat more than half way between the surface ofgyro block 102 and passage 110. Electrode aperture 104 intersectspassage 110 preferably at a right angle. Terminal chamber 125 is formedbetween the surface of gyro block 102 upon which is positioned mirrorsubstrate 140. Terminal chamber 125 is cylindrical in shape having adiameter at least twice that of passage 110. Terminal chamber 125 andpassage 110 are coaxial with one another. Because passage 110 extendsslightly beyond electrode aperture 104 before intersecting with terminalchamber 125, a baffle 145 is formed between electrode 104 and terminalchamber 125.

In prior art system, no terminal chamber or baffle was provided. Thepassage way extended directly through the electrode apertures out to thesurface of the laser gyro block. When the electrodes were excited, dustor other unwanted particles which may be produced such as by ionbombardment and sputtering of the laser gyro block would collect aroundthe intersection of the electrode aperture and beam passageways. Thesuspended particles acted as scattering centers increasing the opticalloss of the structure. In contrast, with the present invention it hasbeen found that dust or other unwanted particles will not be suspendedin the region of the intersection of electrode apertures 104 and passage110. Thus, a potential source of drift is eliminated.

This concludes the description of the preferred embodiments of theinvention. Although preferred embodiments have been disclosed, it isbelieved that numerous modifications and alterations thereto would beapparent to one having ordinary skill in the art without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A Faraday rotator element for a laser gyroscopecomprising in combination:a base support member having an aperturetherein; a permanent magnet having an aperture therein, said aperture insaid permanent magnet being aligned with said aperture in said basesupport member; a slab of rare earth-doped glass, said slab beingpositioned within said aperture in said permanent magnet, said slab andsaid permanent magnet being supported by said base member; a spring,said spring holding said slab against said base member; and a retainingmember for said spring, said retaining member having an aperture thereinaligned with said apertures in said base support member and saidpermanent magnet.
 2. The combination of claim 1 wherein said springcomprises:a coil spring.
 3. The combination of claim 1 wherein:saidpermanent magnet and said retaining member have substantiallycylindrical inner and outer surfaces.
 4. The combination of claim 3wherein said retaining member comprises:a ferromagnetic material.
 5. TheFaraday rotator element of claim 1 wherein said base support membercomprises:a substantially cylindrical central portion; a substantiallycylindrical first end portion having a diameter greater than that ofsaid central portion; and a second end portion, said aperture in saidbase number being in said second end portion, said second end portionhaving a substantially flat portion upon which is mounted said permanentmagnet and said slab, and the longitudinal axes of said aperture in saidbase member and said permanent magnet being substantially perpendicularto the longitudinal axis of said central portion of said base member. 6.The Faraday rotator of claim 5 wherein:the cross-section of said secondend portion, with said permanent magnet and slab mounted thereon, is nogreater than the cross-section of said central portion of said basemember.